Gas sensor assemblies of this type are known for the detection of a broad range of analytes, for example carbon dioxide or methane. Conventional gas sensors, as disclosed, for example, in DE 10 2004 028 077.0, are based on the property of many polar gases to absorb radiation in the infrared wavelength range. Gases of this type consist of two different kinds of atoms such as CO2, but also CO and NOx, and all hydrocarbons such as methane, propane or other natural gases used for heating.
The IR light is able, by cooperating with the dipole moment of the polar molecule, to stimulate the molecules by stimulating rotational and vibratory oscillations. The heat energy of the IR light is thus transmitted to the gas and, in the same way, the intensity of an IR beam passing through a gas volume is reduced. Absorption takes place, in accordance with the stimulated states, at a respective wavelength characteristic of the gas in question—in the case of CO2, for example, at 4.24 μm.
It is therefore possible, using an infrared gas sensor of this type, to establish the presence of a gas component and/or the concentration of this gas component in a test gas. Gas sensors of this type comprise a radiation source, an absorption path, i.e. a measurement chamber in which the gas to be detected is contained, and a radiation detector. The radiation intensity measured by the radiation detector is, according to Lambert-Beer's Law, an indicator of the concentration of the absorbing gas as expressed by the following equation:I=I0 exp(−kcl)wherein I denotes the measured intensity, I0 the irradiated intensity, k a constant, c the concentration of the corresponding gas in molecules per unit of volume, and 1 the length of the measurement path.
In the case of what are known as NDIR (non dispersive infrared) sensors, a broadband IR source is conventionally used as the radiation source and the relevant wavelength is adjusted via an interference filter or screen. Alternatively, a selective radiation source, for example a light-emitting diode or a laser, may also be used in combination with non-wavelength-sensitive radiation receivers.
Carbon dioxide detection, in particular, is becoming increasingly important in a large number of fields of application. For example, the quality of internal air may be monitored both in relation to the operation of motor vehicles. Also, the cleaning cycles of self-cleaning ovens and the feeding of plants with CO2 in greenhouses may be regulated. In the medical field, for example in anaesthetics, the air inhaled by a patient may be monitored and, finally, a carbon dioxide sensor may be used in an associated warning system wherever there is a risk of CO2 escaping, for example in the context of correspondingly filled air-conditioning systems.
In automotive engineering, carbon dioxide detection may be used, in order to increase energy efficiency during heating and air conditioning, to monitor the CO2 content of the internal air in order, if required, i.e. in the event of a high CO2 concentration, to cause a supply of fresh air via a corresponding fan shutter activation means. In addition, modern vehicle air-conditioning systems are based on CO2 as the coolant, so in automotive engineering CO2 gas sensors may also perform a monitoring function in relation to CO2 escaping in the event of any defects. In automotive engineering, in particular, gas sensors of this type have to meet extremely stringent robustness, reliability and miniaturization requirements.
The radiation source of known gas sensor assemblies is often not operated continuously, but rather pulsed at a specific frequency. A constant frequency and a specific pulse-duty ratio is usually selected, the pulse-duty ratio designating the ratio of the on-time (pulse width) to the period time. Disturbances may be reduced by using, during signal processing in the detector region, a narrow-band filter, the filter frequency of which corresponds to the pulse frequency at which the radiation source is pulsed.
As described in DE 10 2004 028 077.0, gas sensor assemblies in which the radiation source is pulsed have the problem, both during start-up of the system and in operating modes in which the radiation sources do not emit any light for a relatively long period of time, that the settling time, i.e. the time before usable test results are available, is comparatively long. In an infrared-based gas sensor operated in a pulsed manner, for example, the system therefore has to settle thermally for such a long time that the first 10 to 15 measured values are unusable. In current designs, it takes from approximately 5 to 10 seconds after start-up until a first reliable measured value is available. However, this is problematic in relation to safety applications, in particular in cases in which the system has to be switched on and off relatively frequently.